Iron-based superconductors La[O 1 - x F x ]FeAs Kamihara et al. - - PowerPoint PPT Presentation

Hyperfine interactions in superconducting KFe2Se2

• I. Madjarevica, V. Koteskia, V. Ivanovskia, C. Petrovicb

a Laboratory of Nuclear and Plasma Physics, University of Belgrade,

Vinča Institute of Nuclear Sciences, 11001 Belgrade, Serbia

b Condensed Matter Physics and Materials Science Department,

Brookhaven National Laboratory, Upton, New York 11973, USA 14th Young Researchers’ Conference – Materials Science and Engineering

Iron-based superconductors

 La[O1-xFx]FeAs – Kamihara et al. [2008] 1 → superconductivity + magnetism !!!!

• layered structure based on a square planar Fe2+ layer

tetrahedrally coordinated pnictogen (P, As)

• r chalcogen (S, Se, T

e) anions.  Classes:

• 1. Doped RE 1111 Fe-pnictide, T

c ~ 25–55K,

• 2. Doped A 122 (A = Ba,Sr)(Ba1-xKxFe2As2), T

c ~ 38 K.

• 3. 111 systems (Li1-xFeAs), T

c ~ 18 K,

• 4. (Sr, Ca, Eu)F FeAs, T

c ~ 36 K,

• 5. Sr4(Sc, V)2O6Fe2(P, As)2, T

c ~ 17 K,

• 6. FeSex, FeSe1-xT

ex with T

c up to 14K,

• 7. AxFe2-ySe2 (A = Tl,K,Rb,Cs) with TC up to 32K.

AxFe2-ySe2 “enigma”

• Guo et al. [2010] 2 :

“SC above 30 K is due to this FeSe-based 122 phase”

• structure I4/mmm (ThCr2Si2 - type)
• a = b=3.9136(1) Å,

c=14.0367(7) Å

• Fe in 4d site
• normal state resistivity very large

→ Antiferomagnetism (AFM) TN ≈ 559 K large moment ~ 3.3 µB/Fe → Fe vacancies !!!

• “strange” phase separation ???
• iron-based SC theory brake-down? 26-28

made with “XCrySDen” software:

• A. Kokalj, Comp. Mater. Sci., 2003, Vol. 28, p. 155.

KxFe2-ySe2 - phase separation

• multiple experimental evidences:

XRD3,4,5, Raman spect.6 Neutron diff.7,8,9,10,11,12 SEM3, TEM13,14 Mössbauer15,16,17 EXAFS18

• two distinctive phases???
• I4/mmm (122) SC phase (disordered

Fe-vacancies) Fe site – 4d

• metallic
• I4/m (245) AFM phase

( 5 × 5 × 1 Fe-vacancies structure equivalent to K2Fe4Se5) Fe sites – 16i and “empty” 4d

• “Mott insulator”

Shoemaker et al. 19

Phase separation revisited

• mesoscopic phase separation (~100 nm)

→ more phases? → AF parent of SC phase?

Ding et al. 3 Back-scattered electron images of SEM measurements on the cleaved surface of three typical samples Yang et al. 20 Schematic top view of four possible magnetic orders in the Fe-Fe square layer with one quarter Fe-vacancies ordered in rhombus

𝟑 × 𝟑

Methods: Mössbauer spectroscopy

 nuclear method in material science

• 57Fe (Eγ = 14.4 keV) source accelerated through a range of velocities
• 1mm/s = 48.075 neV
• fine speed tuning → resonant absorption on sample
• absorption spectra → get hyperfine interactions information relative to α-Fe
• local magnetic field on Fe site
• electric field gradient - EFG
• isomer shift - IS

Picture from: www.helmholtz-berlin.de

Methods: Calculations

 WIEN2k LAPW + local orbitals (lo) method GGA-PBE structure optimization  KFe2Se2

• I4/mmm phase without vacancies

(collinear AFM)

• I4/m phase with two vacancies
• rders:

𝟔 × 𝟔 and 𝟑 × 𝟑 (block AFM)  Comparing case RbFe2Se2

• I4/mmm phase without vacancies

(collinear AFM)

• I4/m phase with vacancies
• rder:

𝟔 × 𝟔 (block AFM)

• charged +1 I4/m phase with

vacancies order: 𝟔 × 𝟔 (block AFM) 𝟔 × 𝟔 𝟑 × 𝟑

Sample preparation

 KFe2Se2 single crystals of were grown by the self-flux method21

• nominal composition K0.8Fe2Se2
• prereacted FeSe and K pieces (purity 99.999%, Alfa Aesar) were

put into the alumina crucible and sealed into the quartz tube under a partial pressure of argon.

• the quartz tube was heated to 1030 °C, kept at this temperature

for 3 h, and then slowly cooled to 730 °C at a rate of 6 °C/h.

• platelike crystals up to 5×5×1mm3 were grown.

 XRD and SQUID measurements where done to check the structure and SC.

Results:

Mössbauer spectroscopy of KFe2Se2

 spectra recorded with Wissel Mössbauer system on room temperature 

57Co Mössbauer source in Rh-matrix (50 mCi / 1.85 GBq)

 textured sample (deviation from 3:2:1:1:2:3 sextet ratio)  fitted subspectras with WinNormos-for-Igor

Results: Calculations

KFe2Se2 RbFe2Se2

I4/mmm I4/m 𝟔 × 𝟔 I4/m 𝟑 × 𝟑 I4/mmm I4/m 𝟔 × 𝟔 I4/m 𝟔 × 𝟔 charged +1 HFF [T] 18.6 22 17 22 18.4 21.8 22.9 EFG [V/m2]

• 0.51 x 1021

2.79 x 1021 4 x 1021 1 x 1021

• 0.43 x 1021

2.80 x 1021 3.30 x 1021 η 0.06 0.85 0.80 0.78 0.40 0.87 0.73 MM 2.39 µB 2.92 µB 2.71 µB 2.85 µB 2.41 µB 2.89 µB 2.96 µB Atomic distances [Å]

Fe-Se: 2.421 Fe-Fe: 2.760 Se-K: 3.455 Fe-Se: 2.414 Fe-Se: 2.445 Fe-Se: 2.452 Fe-Se: 2.511 Fe-Fe: 2.695 Fe-Fe: 2.919 Fe-Se: 2.424 Fe-Fe: 2.771 Se-Rb: 3.502 Fe-Se: 2.409 Fe-Se: 2.425 Fe-Se: 2.432 Fe-Se: 2.491 Fe-Fe: 2.690 Fe-Fe: 2.921 Fe-Se: 2.405 Fe-Se: 2.429 Fe-Se: 2.433 Fe-Se: 2.493 Fe-Fe: 2.684 Fe-Fe: 2.924

Summary

 confirmed “phase separation”  Mössbauer spectroscopy can be used to detect multiple phases in AFe2Se2 → two magnetic Fe-sites on room temperature HFF: 14 T and 27 T → one PM site  this is in accordance with multiple neutron diffraction measurements 7,9,11  hyperfine parameters from the calculations are in good agreement with Mössbauer spectroscopy ( 𝟔 × 𝟔 and 𝟑 × 𝟑)  𝟑 × 𝟑 AFM the best candidate for parent of a SC phase  good agreement with previous calculation works 22,23  excellent agreement of bondlengts values with EXAFS experiment18 → Fe–Se bondlengths highly covalent → Fe–Fe bondlength with much smaller force constant compared to the binary FeSe.  local relaxation of the Fe–Fe bondlength → compression of the FeSe unit  anisotropy in KFe2Se2 results from the Fe-vacancies.  the KxFe2Se2 recalls the oxygen ordering effects on the superconductivity of cuprates 24,25

Thank You !!!

References

[1] Komihara et al., J. Am. Chem. Soc. 2008, 130, 3296-3297 [2] Guo et al., Phys. Rev. B 82, 180520(R) 2010 [3] Ding et al., Nature Communications 4, 1897, (2013). [4] Ricci et al., Supercond. Sci. T

• echnol. 24 082002 (2011)

[5] Liu et al., Phys. Rev. B 92, 059901 (2015) [6] Lazarević et al., Phys. Rev. B 86, 054503 (2012) [7] Sabrowsky et al., J. Magn. and Magne Materials 54 57 (1986) 1497-1498 [8] Bao et al., Chin. Phys. Lett. Vol. 28, No. 8 (2011) 086104 [9] Zhao et al., PRL 109, 267003 (2012) [10] Mittal et al., Phys. Rev. B 87, 184502 (2013) [11] Pomjakushin et al., J. Phys.: Condens. Matter 23 (2011) 156003 (4pp) [12] Ye et al., PRL 107, 137003 (2011) [13] Chen et al., Phys. Rev. X 1, 021020 (2011) [14] Li et al., Nature Physics 8, 126–130 (2012) [15] Nowik et al., Supercond. Sci. T

• echnol. 24 (2011) 095015 (6pp)

[16] Ryan et al., Phys. Rev. B 83, 104526 (2011) [17] Ksenofontov et al., Phys. Rev. B 84, 180508(R) (2011) [18] Iadecola et al., J. Phys.: Condens. Matter 24 (2012) 115701 (6pp) [19] Shoemaker et al., Phys. Rev. B 86 184511 (2012) [20] Yang et al., PRL 106, 087005 (2011) [21] Hechang Lei and C. Petrovic., Phys. Rev. B 83, 184504 (2011) [22] Yan et al., PRL 106, 087005 (2011) [23] C. Cao and J. Dai, PRL 107, 056401 (2011) [24] Fratini et al., Nature 466 841 (2010) [25] Poccia et al., J. Supercond. Novel Magn. 24 1195–200 (2011) [26] Qian T et al.,Phys. Rev. Lett. 106 187001 (2011) [27] Zhang Y et al Nat. Mater. 10 273 (2011) [28] Zhao et al., Phys. Rev. B 83 140508 (2011)